Stiffener

A stiffener is a plate or rolled section welded to the web of a steel beam or column to prevent local buckling, distribute concentrated loads, or resist out-of-plane bending. The Indian standard IS 800:2007 Cl. 8.7 governs stiffener design for I-section beams and plate girders; Cl. 8.7.4 covers bearing stiffeners at supports; Cl. 8.7.5 covers intermediate stiffeners along the span; and bearing-type stiffeners at concentrated loads are detailed in Cl. 8.7.4.

Four principal types: (1) Bearing stiffeners — at beam supports, transferring concentrated reaction force from the bottom flange through the web to the top flange; required when the reaction load exceeds the web's local bearing capacity. (2) Intermediate stiffeners — along the beam span at intervals, preventing web shear buckling. Spacing per Cl. 8.7.5 depends on the web's depth-to-thickness ratio and applied shear. (3) Diagonal stiffeners — at beam-column joints, resisting joint shear in moment-resisting frames per IS 13920 ductile detailing. (4) Bearing-type stiffeners at concentrated loads — under columns landing on beams, under heavy equipment, or at points of concentrated load application; sized to transfer the local load through the web.

Design considerations per IS 800:2007 Cl. 8.7: (a) stiffener cross-section sized for stress on a column-section idealization; (b) stiffener fillet-welded to web on both sides; (c) stiffener fitted (or fillet-welded) to the loaded flange; (d) stiffener clipped at the unloaded flange to allow weld access. The most common Indian fabrication issue is incomplete stiffener-to-flange contact — gap of 2-3 mm common, requiring shim plates or oversized fillet welds. Pre-fabrication, the stiffener should be sawn / planed to match the flange profile precisely. Site engineers should refuse acceptance of stiffeners with visible gaps to flanges.